alloc_netdev() and 32 byte or 32 bit alignment???

[Eric Dumazet]

On Fri, 18 May 2007 17:10:02 +0530
"pradeep singh" <2500.pradeep at gmail.com> wrote:

> Hi ,
> 
> following code snippet is self evident in what it does -
> 
> truct net_device *alloc_netdev(int sizeof_priv, const char *name,
> 		void (*setup)(struct net_device *))
> {
> 	void *p;
> 	struct net_device *dev;
> 	int alloc_size;
> 
> 	/* ensure 32-byte alignment of both the device and private area */
> 	alloc_size = (sizeof(*dev) + NETDEV_ALIGN_CONST) & ~NETDEV_ALIGN_CONST;
> 	alloc_size += sizeof_priv + NETDEV_ALIGN_CONST;
> 
> 	p = kzalloc(alloc_size, GFP_KERNEL);
> 	if (!p) {
> 		printk(KERN_ERR "alloc_dev: Unable to allocate device.\n");
> 		return NULL;
> 	}
> [snip]
> 
> Is the comment correct?

yes it seems correct

> Why is it 'ensure 32 byte alignment', instead of 32 bit alignment?

Its 32 bytes, not bits.

#define NETDEV_ALIGN        32
#define NETDEV_ALIGN_CONST  (NETDEV_ALIGN - 1)

> 
> What am i missing here?

I dont know :)

This 32 bytes alignement maybe not up2date, because net_device contains several ____cacheline_aligned_in_smp

Would be nice to switch the alignement to max(L1_CACHE_SIZE, 32) nowadays, at least on SMP